1. Field of the Invention
The present invention relates generally to processes and systems for producing fullerenes. More particularly, the present invention relates to the production of fullerenes utilizing the basic physical vapor deposition process commonly known as sputtering.
2. Description of Related Art
The publications and other reference materials referred to herein to describe the background of the invention and to provide additional details regarding its practice are hereby incorporated by reference. For convenience, the reference materials are numerically referenced and identified in the appended bibliography.
Fullerenes are cage-like molecules which constitute the third form of pure carbon. The other two pure forms of carbon are diamond and graphite. C.sub.60 was one of the first fullerenes to be produced in gram quantities. The initial processes for producing C.sub.60 involve the use of resistive (1, 2) or arc heating (3) of graphite. The availability of gram quantities of C.sub.60 has lead to a period of intense ongoing research into the chemical, physical and material properties of this first molecular allotrope of carbon (4, 5).
More recently, many additional fullerenes have been isolated. C.sub.70, C.sub.76, C.sub.78, C.sub.84, C.sub.90, C.sub.94 and C.sub.96 are just a few of the additional fullerenes which have been identified (6). These fullerenes have been commonly referred to as the "higher" fullerenes.
International Patent Application No. WO92/04279 published on Mar. 19, 1992, discloses an exemplary method for producing fullerenes. This method, like the above-described methods involves the resistive or arc heating of graphite in the presence of an inert quenching gas to form a black soot material which contains fullerenes. As set forth in this published patent application, C.sub.60 is the predominant fullerene produced in the process where carbon is evaporated in the presence of an inert quenching gas. When the pressure of the inert gas is on the order of 100 Torr, the fullerene product isolated from the soot contains 60 to 70 weight % C.sub.60, 20 to 30% C.sub.70 and approximately 5 weight % of higher fullerenes in the range between C.sub.76 and C96 (5- pp. 119-126).
Although the above procedures are capable of producing relatively large amounts of C.sub.60 and C.sub.70, the isolation of higher fullerenes, e.g. C.sub.76, C.sub.78 and C.sub.78 from these mixtures is tedious and yields only limited amounts of pure materials. Since investigations now show that the chemistry of higher fullerenes promises to be diverse and distinctly different from the chemistry of C.sub.60 and C.sub.70, it would be desirable to provide processes and systems for preparing the higher fullerenes in larger quantities (7).
Laser vaporization of a rotating graphite target in a tube furnace (5- pp. 98-105) and inductive heating of graphite powder (8) have also been used in fullerene production processes. Both of these vaporization methods also yield soot in which C.sub.60 is by far the predominant product. Also, C.sub.60 and C.sub.70 have been isolated from the soot produced in oxidizing benzene flames. Depending upon the conditions, variations of the C.sub.70 /C.sub.60 ratio range from about 0.26 to 5.27 (9).
In view of the continuing significant interest in fullerenes, there is a continuing need to provide processes and systems for producing a variety of fullerenes in useful amounts which can be used for further study and evaluation. Further, there is a need to provide processes and systems which are capable of producing fullerene mixtures which are especially rich in the higher fullerenes, i.e. C.sub.76 -C.sub.96.